The invention relates to a filter unit for connectors, comprising a substrate of electrically insulating material which has two flat sides lying opposite each other and is provided with passages for the contact elements of the connector, capacitors being disposed on one flat side of the substrate in the region of one or more of the passages and being made up of first electrodes formed by at least one layer of electrically conducting material which extends over said side of the substrate and is provided with correspondingly situated larger passages, second electrodes formed by spaced-apart electrode patches of electrically conducting material which cover said passages of said substrate and can be connected to the contact elements of the connector, and at least one layer of dielectric material extending between the first and second electrodes in such a way that the passages are open.
A filter unit of this type is known from European Patent Application EP-A-123457.
In electrical transmission technology pulse-type signals are being used to an increasing extent for the transmission of data. As is known in electrical engineering, pulse-type signals can be broken down into a series of sinusoidal signals with increasing frequency, the so-called higher harmonics. In signals with a high pulse frequency, which are usual in computers, higher harmonics in the megahertz and even up to the gigahertz range can occur.
The steepness of the pulse edges, called the rise time, also plays an important role. A usual rise time of one nanosecond already corresponds to a higher harmonic frequency of about 350 MHz, irrespective of the pulse frequency itself.
These higher harmonics are found to cause great interference. In a room in which there are several interconnected electronic processing units producing pulse-type signals, the higher harmonics readily cause interference in the data processing. This interference can become so great that proper functioning of, for example, computers is no longer possible.
In order to keep the total interference level to a minimum, it is necessary to use filters by means of which the undesirable higher harmonic frequencies can be damped, without the desired data signal being deformed too much. A capacitor is a suitable element for this purpose, because the reactance thereof is inversely proportional to the frequency. This means that the reactance is greater for relatively low frequencies than for higher frequencies.
With the known filter unit each of the contact elements of a connector can be decoupled to earth by means of a capacitor. The filter unit is produced by the so-called thick film silkscreen printing technique on a flat substrate, so that capacitors with sufficiently low inductance can be produced cheaply for the effective damping of signals at high frequencies. The capacitance value of the flat capacitors thus formed is directly proportional to the surface area of the electrodes lying opposite and the relative dielectric constant of the dielectric between them, but is inversely proportional to the distance between the electrodes.
The disadvantage of the known filter unit is that the capacitance value of the filter capacitors formed therewith is limited by the space available on the one side of the substrate for the electrode patches surrounding the passages. The available surface area for an electrode patch is essentially determined here by the distance between the passages, which of course corresponds to the pitch of the connecting elements of the connector. For the arrow-shaped electrode patch of the known filter unit, the one pointed end of which surrounds the passage, while the other broad end extends towards the edge of the substrate, particularly with small pitches of the order of 2 mm and with more than two-row connectors, which are in great demand in the art, too little surface area is available to obtain that capacitance value which is necessary for good filtering.